This invention relates to boilers. In one aspect, the invention relates to a boiler in combination with a fuel cell while in another aspect, the invention relates to the use of water used to cool the fuel cell as a source of boiler feedwater heating. In another aspect, the invention relates to an integrated boiler/fuel cell system useful for cogenerating steam at various pressures and electricity.
Fuel cells are known to be useful for the cogeneration of electricity and thermal energy (i.e. heat), and they are available in a variety of configurations. Representative fuel cell types include phosphoric acid fuel cells (PAFC), molten carbonate fuel cells (MCFC), solid oxide fuel cells (SOFC), polymer electrolyte fuel cells (PEFC), and alkaline fuel cells (AFC). These cells and their operation are described in Fuel Cells: A Handbook, May 1988, published by the United States Department of Energy, which is incorporated herein by reference.
Fuel cells, regardless of their configuration, are designed to produce electrical power. However due to thermodynamic theory and for practical reasons, they also produce thermal energy which must be transferred from the cell (the reactant gases and the cell structure) as heat. Since fuel cells operate best at or near a specified temperature, typically the cell unit is designed with a means for removing this heat. Fluids, such as air and/or water, are generally used as the cooling agent.
In those systems in which water is used as the cooling agent, heat is transferred from the fuel cell system and its reactant gases to water, and subsequently from this water to another fluid (e.g. a mixture of glycol, propylene and water). The thermal energy in this mixture is then transferred as heat in another process, e.g. heating water. While such systems are generally effective for producing hot water (the temperature of which is typically between 150-180 F.) , these systems are not capable of or effective at producing steam at various pressures.
Boilers are designed to produce steam, and the water that they convert to steam is known as, among other names, boiler feedwater. This feedwater can enter the boiler at virtually any temperature between ambient and the boiling temperature, however boiler operating efficiencies are enhanced if the feedwater enters the boiler at a temperature approaching the boiling temperature. In many present day boiler operations, the feedwater is preheated to within a desirable temperature range through the use of a fuel-fired burner and/or steam extraction.